It is known that in high-rate digital data transmission the degradation of the signal is mainly due to the so-called "intersymbol interference". This is caused by the fact that, owing to non-ideal characteristics of the transmission channel, every pulse containing the digital information is associated with so-called postcursors (tails) and precursors which overlap and distort adjacent pulses.
Such interference, if too strong, makes the decision on the transmitted signal incorrect; besides, it increases as transmission rate increases, so as to represent the biggest obstacle to the rate increase, thus preventing a cheaper exploitation of the transmission lines.
At the receiving side, it is therefore necessary to have devices able to compensate the harmful effects of intersymbol interference, both for the signal portion representing the precursors of the pulses following the one on which the decision is taken, and for the signal portion representing the postcursors of the preceding pulses.
Such compensation is obtained by the use of suitable filters, known as equalizers.
It is well known in the literature that favorable results in the equalization with respect to interference phenomena due to postcursors have been attained by utilizing nonlinear structures, for instance those of the decision-feedback type.
For the equalization with respect to phenomena due to the precursors a correction of linear type is usual.
The combined use of the two techniques allows to obtain better results than those attainable by only linear equalization, but it does not allow high transmission rates, with acceptable performance; in fact, at such rates the degradation component due to precursors considerably increases, and it is difficult to be compensated by linear equalization, which also causes an increase of the noise power degrading the performance.
As the present digital-transmission techniques are developing systems with ever higher transmission rate, researches aiming for the realization of an equalizer embodying the principle of precursor compensation through decision feedback are becoming more and more important.
A solution of this problem has been given by R. T. Boyd and F. C. Monds in a paper entitled "Equalizer for digital communication" (Electronics Letters, 28 Jan. 1971, Vol. 7, No. 2, pp. 58-60).
This paper describes a nonlinear equalizer with decision feedback, in which an iterative correction of errors due to symbol precursors and postcursors is achieved through transversal filters.
The practical embodiment of such a system is rather difficult because, in order to obtain a good correction of postcursor errors, it is necessary to repeat more than once the decision feedback as the precursor-error probability is reduced; that obviously entails an iteration of the transversal filter performing such feedback.
Besides, the described system is not suitable for use in the equalization of phase-modulated signals, for which further filters must be used to compensate the interference caused by the quadrature components of the signal (interchannel interference), with consequent increased complexity.